Photosensitive glass paste and method for manufacturing multilayered interconnected circuit board using the same

ABSTRACT

A photosensitive glass paste containing: an inorganic component containing a glass powder and a photosensitive organic component is provided. The glass powder contains about 1 to 30 percent by weight of a low melting point glass powder having a glass softening point in a range of about 400° C. to 600° C. and about 70 to 99 percent by weight of a high melting point glass powder having a glass softening point about 300° C. or more higher than the glass softening point of the low melting point glass. As the high melting point glass, a borosilicate glass which inhibits diffusion of a conductive component such as Ag is employed. The content of the inorganic component in the photosensitive glass paste is in a range of about 40 to 70 percent by weight. A method of manufacturing a multilayered interconnected circuit board using the above-described photosensitive glass paste is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a photosensitive glass pastewhich is cured by exposure to light, particularly for making aninsulation layer in multilayered electronic device, and to a method formanufacturing a multilayered interconnected circuit board using theglass paste.

2. Description of the Related Art

Conventionally, semiconductive elements, such as ICs, etc., are usedmounted on a printed circuit board in which an insulating layer having athrough hole is formed by processes such as printing processes, etc., ona glass epoxy resin board having fine wiring on the surface thereof.

In recent years, as the demand for high integration, fineinterconnection, high speed transmission, high frequency and rapid heatdissipation increases for semiconductor elements, the demand for aprinted circuit board capable of accommodating such needs has alsoincreased.

The conventional printed circuit board, however, does not havesatisfactory characteristics regarding workability, platability ofthrough holes, adhesive property among layers, etc., resulting inproblems. Also, there is a problem of thermal deformation at hightemperatures in the above described glass epoxy resin board and there isa limit to increases in density.

In view of the above, a ceramic substrate which uses a ceramic having ahigh mechanical strength and a high thermal resistance, such as alumina,is considered to be desirable.

As a ceramic substrate, there may be mentioned a printed substratehaving fine wiring on the surface thereof and having an insulating layerwith through holes formed by the printing techniques, etc., on thesurface thereof. In this type of printed substrate, it may sometimes benecessary to form fine through holes in order to accommodate highdensity packaging.

Also, in recent years, multilayer-type inductors capacitors, etc.,formed by laminating a ceramic green sheet provided with electrodes(coil patterns) are being increasingly used. As the level of integrationof electronic circuits increases, passive devices such as inductors,capacitors, etc., are also required to be miniaturized.

A spiral multilayer coil capable of yielding high inductance, forexample, is manufactured by repeatedly forming an insulating layer, withvia holes, on a ceramic green sheet (alumina green sheet) provided witha coil pattern, filling a conductor into the via hole, stacking anotherceramic green sheet on the insulating layer, and connecting the coilpattern of the first layer and the coil pattern of the second layer viathe via holes. When a coil is manufactured using this process, it isnecessary to form fine through holes in order to achieve miniaturizationof the product.

In response to the trends to higher density and miniaturization of themultilayer circuit components, a method of forming an insulating layerby using a photosensitive glass paste has been proposed, by which finevia holes which have been difficult to form by conventional printingtechniques can be formed.

In this method, a photosensitive glass paste is applied on the entiresurface of the substrate by way of screen-printing, etc. The appliedphotosensitive glass paste is dried, is then exposed and developedthrough a photomask, and is fired so as to form the fine via holes.

It is to be noted that, the photosensitive glass paste used in thismethod is a mixture of a photosensitive organic material and aninorganic mixture containing a glass or both a glass and a ceramic. Asthe inorganic mixture, a mixture in which ceramic material such asquartz, cordierite, alumina, zirconia, mullite, spinel, forsterite andsilica are added to the glass at a predetermined proportion is generallyused. As the photosensitive organic material, one including a polymerhaving a functional group in the side chain thereof, a photoreactivecompound (monomer), a photopolymerization initiator, a solvent, etc., isgenerally used.

The conventional photosensitive glass paste, however, usually containsonly one kind of glass component having a relatively low glass softeningpoint. Thus, viscous flow is initiated at a low temperature, resultingin a large shrinkage ratio due to sintering and the diameter of theformed via hole will be significantly larger than the diameter afterdevelopment. Another problem is that during the step of firing, theamount of Ag from the conductive leads made of Ag which constitute thecircuit diffusing into the glass is large, degrading insulatingcharacteristics between the layers.

In contrast, when the glass component of the photosensitive glass pasteincludes only a high melting point glass, as the photosensitive glasspaste is treated with heat in the range of the optimum firingtemperature so as not to cause deformation of conductive leads made ofAg and Cu, there are problems of insufficient sintering and degradationof insulative properties.

Japanese Unexamined Patent Application Publication 10-120432 discloses aphotosensitive glass paste for use in plasma displays, in which a highmelting point glass is added to a low melting point glass. Because thecontent of the low melting point glass is high (40 to 97 percent byweight), when this photosensitive glass paste is employed in amultilayered interconnected circuit board, deformations of via holes dueto shrinkage cannot be avoided. Also, because viscous flow of the lowtemperature glass occurs at low temperatures, a conductor such as Agwill diffuse, causing the insulation properties between layers todegrade. Furthermore, because the functional group, such as carboxylgroup, in the photosensitive organic component reacts with polyvalentmetal such as boron contained in the glass component, the polymer chainsare cross-linked and the viscosity of the paste is abnormally increased,resulting in gelation.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aphotosensitive glass paste which is not susceptible to gelation,shrinkage due to sintering and diffusion of conductor compositions suchas Ag, by which a glass layer having via holes of precise predeterminedshape and size can be formed at predetermined positions. Another objectof the present invention is to provide a method for manufacturing amultilayered interconnected circuit board using the photosensitive glasspaste.

In order to achieve the above objects, a photosensitive glass paste isprovided comprising an inorganic component including a glass powder, anda photosensitive organic component, in which the glass powder comprises(a) about 1 to 30 percent by weight of a low melting point glass powderhaving a glass softening point in a range of about 400° C. to 600° C.and (b) about 70 to 99 percent by weight of a high melting point glasspowder having a glass softening point about 300° C. or more higher thanthe softening point of the low melting point glass powder.

Because the glass component which constitutes the main portion of theinorganic component of the photosensitive glass paste of the presentinvention contains a high melting point glass as the main componentthereof and limits the proportion of the low melting point glass to beless, shrinkage due to sintering, diffusion of conductive componentssuch as Ag, and gelation can be prevented.

Also, by using this photosensitive glass paste, for example, a glasslayer having via holes of desired size can be reliably formed.

The photosensitive glass paste of the present invention uses about 1 to30 percent by weight of a low melting point glass powder having a glasssoftening point in a range of about 400° C. to 600° C. The reason forusing this type of glass powder is that when the glass softening pointis lower than about 400° C., diffusion of metal contained in the leadsoccurs, causing defects in the insulating layers (degradation ofinsulating properties), and when the glass softening point exceeds about600° C., the glass paste remains unsintered. The reason for setting theproportion of the low melting point glass powder to about 1 to 30percent by weight is that when the proportion of the low melting pointglass powder is less than about 1 weight percent, sintering is notperformed satisfactorily and bubbles remain in the insulating layerscausing insulation failure between layers of the multilayer circuit, andwhen the proportion of the low melting point glass powder exceeds about30 percent by weight, patterns may be deformed after firing, and metalused in the lead may diffuse into the glass, causing insulation failure.When the proportion of the high melting point glass powder exceeds about99 percent by weight, sintering is not performed satisfactorily andbubbles remain in the insulating layers, causing insulation failurebetween layers of the multilayer circuit.

When a high melting point glass powder which satisfies the conditions ofthe present invention, i.e., the high melting point glass powder havinga glass softening point (T_(S)) of about 300° C. or more higher thanthat of the low melting point glass powder, is used, by firing at atemperature about 10° C. to 100° C. higher, preferably about 20° C. to30° C. higher, than the glass softening point (T_(S)) of the highmelting point glass, the viscosity of the low melting point glass isappropriately decreased, promoting sintering. Thus, sintering can beperformed easily and reliably. In addition, because the viscosity of thehigh melting point glass is not decreased as much at the time of firing,deformation in the shape of the pattern can be prevented.

It is to be noted that when the difference between the glass softeningpoint of the low melting point glass and the glass softening point ofthe high melting point glass is less than about 300° C., the low meltingpoint glass does not flow properly, inhibiting sintering, even when thefiring temperature is set to a temperature about 20° C. to 30° C. higherthan the glass softening point of the high melting point glass. When thefiring temperature is increased so that the low melting point glass mayflow properly, the viscosity of the high melting point glass is reducedand the entire glass paste layer may flow, resulting in deformation ofthe pattern shape.

As described above, a low melting point glass having a glass softeningpoint in a range of about 400° C. to 600° C. is preferably used as thelow melting point glass. More preferably, a low melting point glasshaving a glass softening point in the range of about 450° C. to 550° C.is used.

Preferably, the photosensitive glass paste of the present inventionincludes a high melting point glass powder comprising an SiO₂—B₂O₃—K₂Otype (borosilicate type) glass powder in which the proportion in weightof SiO₂, B₂O₃ and K₂O is in a region surrounded by point A (65, 35, 0),B (65, 25, 10), C (85, 5, 10) and D (85, 15, 0) in a ternary diagramshown in FIG. 1.

In the above described high melting point glass powder, the proportionof SiO₂, which has a low reactivity with an organic binder, is high andthe proportion of the composition (boron, in particular) having a highreactivity with an organic vehicle, particularly with a photosensitiveorganic binder having an acid functional group such as a carboxyl group,is relatively low. Thus, changes in viscosity of the photosensitiveglass paste over time due to ionic cross-linking reactions areprevented, layers can be uniformly formed by using various techniques,and fine via holes, etc., having highly precise forms can be readilyformed by a photolithographic technique.

An insulating body layer formed of a glass paste (insulating material)comprising a borosilicate glass of the above composition as the maincomponent thereof displays superior characteristics, i.e., the relativedielectric constant ∈_(r) is as low as 5 or less and insulationreliability in wet load testing is as high as 1×10⁹ or more (log IR≧9).

Furthermore, because a borosilicate glass capable of inhibiting thediffusion of a conductor component such as Ag is employed as the highmelting point glass, it is possible to reduce the diffusion of metal(conductive component) used in leads and to decrease the conductivity ofthe glass layer. Thus, by using the photosensitive glass paste of thepresent invention, a multilayer electronic component, circuit or thelike having less transmission loss can be formed.

By optimally varying the composition proportion of SiO₂, B₂O₃ and K₂O₃within the region surrounded by points A, B, C, and D in FIG. 1, it ispossible to control the desired thermal expansion coefficientcorresponding to the types of substrate material and conductive materialwithin the range of, for example, 1.5 to 9 ppm/° C. Accordingly, warpageof the substrate may be minimized, the deformation of the substrate maybe decreased when layers are stacked and a thick film multilayeredinterconnected circuit board of high reliability can be obtained.

In region X shown in FIG. 1, the insulating resistance in wet loadtesting is lower and the insulation reliability is likely to decrease.In region Y shown in FIG. 1, the relative dielectric constant ∈_(r) ishigh, and such a glass paste is not suitable for an insulating bodylayer of the thick multilayered interconnected circuit board for highfrequency use. In region Z shown in FIG. 1, the sintering temperaturefor forming insulating layers is increased, and a low melting metalhaving a low specific resistance such as gold and copper will bedifficult to fire simultaneously, thereby decreasing productivity.

Preferably, the composition of the SiO₂—B₂O₃—K₂O type glass is in theregion surrounded by points E (75, 25, 0), F (75, 20, 5), G (85, 10, 5)and D (85, 15, 0) in FIG. 1.

More preferably, a photosensitive glass paste of the present inventionincludes a high melting point glass powder comprising a mixed powdercontaining the above-described SiO₂—B₂O₃—K₂O type glass and aSiO₂—B₂O₃—Al₂O₃ type glass having a composition of about 93.5 to 97.8percent by weight SiO₂, about 2.0 to 5.0 percent by weight B₂O₃ andabout 0.2 to 1.5 percent by weight Al₂O₃.

By using a mixture of a borosilicate glass having a composition withinthe region surrounded by points A, B, C and D in FIG. 1 and aSiO₂—B₂O₃—Al₂O₃ type glass (high silica content silicate glass)comprising about 93.5 to 97.8 percent by weight SiO₂, about 2.0 to 5.0percent by weight B₂O₃ and about 0.2 to 1.5 percent by weight Al₂O₃,shrinkage of the paste due to firing can be inhibited and changes in thepattern shape due to firing can be minimized. Also, by using a highmelting point glass having the above composition, viscous flow of glassis inhibited, via holes are prevented from being enlarged and a glasslayer having a desired pattern may be formed on an insulating layer.

Also, by adjusting the proportions of the above-described borosilicateglass and the high silica content silicate glass, the thermal expansioncoefficient of the insulating body layer and that of the substrate maybe controlled to coincide so as to yield a multilayer circuit substratewith less warpage.

Preferably, the proportion of the high silica content silicate glass isadjusted to be about 15 to 35 percent by weight, and more preferablyabout 20 to 30 percent by weight, relative to 100 percent by weight ofthe borosilicate glass.

The above-described high silica content silicate glass has a high glasssoftening point (T_(S)) and does not soften at the temperature at whichthe photosensitive glass paste, the product, is fired. The high silicacontent silicate glass not only functions as a filler for restrainingshrinkage due to firing but also restrains the scattering of light andenhances the curing rate of the photosensitive glass paste because therefractive index thereof is close to the refractive index of the organicbinder and the borosilicate glass.

Furthermore, because a high silica content silicate glass powder is easyto manufacture and the price thereof is stable, a high silica contentsilicate glass powder of reliable quality can be obtained at a lowercost.

The high silica content silicate glass powder has satisfactorywettability with the above-described borosilicate glass powder. Thus,sinterablilty can be enhanced and a densely sintered body can be formed.

When the above-described high silica content silicate glass has lessSiO₂ than that in the above region, the glass softening point will belower and the refractive index will be higher. Thus, the difference inrefractive index with other materials occurs in some part, scattering oflight in those parts is increased, and the curing rate of thephotosensitive glass paste may be undesirably lowered. When the contentof SiO₂ is higher than the above-described range, vitrification isdifficult, resulting in increased cost.

Preferably, the photosensitive glass paste of the present inventionincludes about 40 to 70 percent by weight of the above-describedinorganic component.

By setting the proportion of the inorganic component to the range ofabout 40 to 70 percent by weight, a photosensitive glass paste ofsuperior quality can be obtained. That is, there is less shrinkage ofthe applied paste due to firing, voids are not significantly formed byfiring, the insulation reliability of the formed layer is high andcuring properties of layers are excellent.

When the inorganic component is less than about 40 percent by weight, itwill be difficult to form a uniform layer due to the significantshrinkage by firing, and insulation reliability is degraded due to anincreased number of voids. When the inorganic component exceeds about 70percent by weight, scattering and absorption of light in the pastelayers are increased at the time of development, causing the amount oflight passing through the layers to be insufficient and a layer curingrate to decrease.

A method for manufacturing a multilayered interconnected circuit boardof the present invention comprises: a step for printing thephotosensitive glass paste on the insulating substrate having conductiveleads formed thereon and drying the applied photosensitive glass paste;a step of forming a via hole pattern by exposing and developing theprinted and dried photosensitive glass paste; and a step of forming aninsulating layer with via holes by filling a conductive paste into thevia hole pattern and firing the same.

Because the glass component which constitutes the main portion of theinorganic component of the photosensitive glass paste of the presentinvention contains a high melting point glass as the main componentthereof and limits the proportion of the low melting point glass, byusing this type of photosensitive glass paste, shrinkage due to firingcan be prevented, diffusion of the conductive component such as Ag canbe inhibited and a thick film multilayer circuit substrate of highreliability can be efficiently manufactured.

Furthermore, because the above-described manufacturing method employsthe photosensitive glass paste of the present invention in order to forman insulating layer with via holes, shrinkage due to firing anddiffusion of conductive component such as Ag can be inhibited. Thus, athick film multilayered interconnected circuit board of high reliabilitycan be efficiently manufactured.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a ternary diagram showing composition regions of borosilicateglass of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail by way ofembodiments.

Preparing Photosensitive Glass Paste

First, a low melting point glass having a glass softening point in arange of about 400° C. to 600° C. is added to a high melting point glasshaving a glass softening point of about 300° C. or more higher than thatof the low melting point glass so that the proportion of the low meltingpoint glass relative to the mixture of ceramic particles and crystallineparticles is about 1 to 30 percent by weight of the entire inorganiccomponent.

As ceramic particles, dielectric ceramics such as BaTiO₃, magneticceramics such as ferrite, and other various types of materials can beemployed. As crystalline particles, quartz can be employed.

The amount of the ceramic particles is preferably about 10 to 25 percentby weight of the entire amount of the inorganic component.

Each of the high melting point glass and low melting point glass mayinclude only one type of glass or a mixture of several types of glass.

The average grain diameter of respective powders such as the highmelting point glass powder, the low melting point glass powder, theceramic particles, and the crystalline particles is in a range of about1 to 10 μm, preferably about 1 to 4 μm.

A photosensitive organic component is then added to the inorganiccomponent at a predetermined proportion and is mixed. The amount of thephotosensitive organic component is controlled so that the proportion ofthe inorganic component is in a range of about 40 to 70 percent byweight.

In this embodiment, a copolymer of methylmethacrylate and methacrylicacid, monomer, photopolymerization initiator and solvent were used.

The mixture of the inorganic component and the photosensitive organiccomponent is thoroughly dispersed by a three-roller-mill to yield aphotosensitive glass paste. Preferably, the photosensitive glass pastecontains about 40 to 70 percent by weight, and more preferably 50 to 55percent by weight of the inorganic component.

Manufacturing Multilayered Interconnected Circuit Board

First, an alumina substrate for a multilayered interconnected circuitboard is prepared. On the alumina substrate, a conductive paste isapplied by screen printing by using a screen sheet having apredetermined pattern, and is then dried and fired so as to form aconductive lead on the surface of the alumina substrate.

A conductive paste comprising metal such as Au, Pt, Ag, Cu, Ni, Pd, W,etc., as a conductive component can be employed, and there is nospecific limitation to the type thereof.

The photosensitive glass paste manufactured as above is then applied onthe entire alumina substrate having a conductive lead by means ofscreen-printing, spin coating, etc., and is dried. The photosensitiveglass paste is then exposed through a photomask having a via holepattern so as to expose part of the conductive lead, and is developed.Then, firing is performed so as to form an insulating layer (film)having via holes of predetermined size and shape at the predeterminedpositions on the alumina substrate having a conductive lead.

The conductive paste is then applied by screen printing on the formedinsulating layer with a desired pattern while filling via holes withconductive paste. In this manner, since the via holes are filled withconductive paste and a conductive lead pattern is formed on theinsulating layer, the circuits of the first layer and the second layerare serially connected. These are then dried and fired in theabove-described conditions.

By repeating the above-described steps, a multilayered interconnectedcircuit board having a predetermined number of layers is manufactured.

EXAMPLES

Now, the present invention will be more specifically described by way ofexamples.

Preparing Photosensitive Glass Paste

As shown in Table 1, a borosilicate glass having a composition ofBi₂O₃:B₂O₃:Al₂O₃:SiO₂=73.9:22.2:3.2:1.4 on a weight basis is prepared asa low melting point glass. Two types of glass having a glass softeningtemperature about 300° C. or more higher than the glass softening pointof the low melting point glass are prepared as high melting pointglasses. The first high melting point glass comprisesSiO₂:B₂O₃:K₂O=79:19:2 and the second high melting point glass comprisesSiO₂:B₂O₃:Al₂O₃=96:2.5:1.4. For the purpose of comparison, a comparativehigh melting point glass in which the ratio by weight ofSiO₂:B₂O₃:Al₂O₃:Na₂O:MgO:CaO=72.6:0.8:1.7:15.2:3.6:4.6 having asoftening temperature 216° C. higher than that of the low melting pointglass is prepared.

TABLE 1 Glass Softening Component Temperature Insulating BorosilicateGlass: First High Melting Point  780° C. Powder Glass (SiO₂:B₂O₃:K₂O =79:19:2) Borosilicate Glass: Second High Melting Point 1500° C. Glass(SiO₂:B₂O₃:Al₂O₃ = 96:2.5:1.4) Corning 0080: Comparative High MeltingPoint  696° C. Glass (SiO₂:B₂O₃:Al₂O₃:Na₂O:MgO:CaO =72.6:0.8:1.7:15.2:3.6:4.6) Borosilicate Glass: Low Melting Point Glass 480° C. (Bi₂O₃:B₂O₃:Al₂O₃:SiO₂ = 73.9:22.2:3.2:1:4) Quartz —

The glass softening point of each glass material is also shown in Table1.

The low melting point glass is added to these high melting point glassesor to the comparative high melting point glass at a proportion of 0.6 to35 percent by weight, as shown in Table 2. Quartz is added so as toadjust the mixture of the inorganic component.

TABLE 2 Amount of Amount of Ratio by weight Amount of First Second HighComparative Amount of Low of High Melting High Melting Melting HighMelting Melting Point Amount of Point Glass to Point Glass Point GlassPoint Glass Glass Quartz the Low Melting Sample (in weight %) (in weight%) (in weight %) (in weight %) (in weight %) Point Glass A* 65.6 17.9 —0.5 16 99.4:0.6 B 65.2 17.8 — 1.0 16 98.8:1.2 C 64.0 17.5 — 2.5 1697.0:3.0 D* — 17.5 64.0 2.5 16 97.0:3.0 E 81.5 — — 2.5 16 97.0:3.0 F* —— 81.5 2.5 16 97.0:3.0 G 54.2 14.8 — 15.0  16  82.1:17.9 H 46.2 12.6 —25.2  16  70.0:30.0 I* 38.5 10.5 — 35.0  16  58.3:41.7 J* 0 0 — 80   20  0:100

It is to be noted that the average grain diameters of the first highmelting point glass, the second high melting point glass, thecomparative high melting point glass, the low melting point glass, andquartz were 4 μm, 3 μm, 3 μm, 1 μm, and 2 μm, respectively

In Table 2, asterisked samples A, D, F, I, and J are the samples ofcomparative examples having compositions outside the range of thepresent invention.

A photosensitive glass paste was adjusted by mixing each of thematerials below in the proportions below.

(a) Copolymer of methylmethacrylate and methacrylic acid: 7 parts byweight (14 percent by weight)

(b) Monomer (ethylene oxide modified trimethylolpropane triacrylate): 14parts by weight (28 percent by weight)

(c) Initiator(2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one): 2 partsby weight (4 percent by weight)

(d) Solvent (ethylcarbitol acetate): 27 parts by weight (54 percent byweight)

Then, as shown in Table 3, 50 parts by weight of the inorganic component(insulating powder) of Table 2 and 50 parts by weight of the abovedescribed photosensitive organic component are mixed thoroughly by athree roller mill to manufacture a photosensitive glass paste.

TABLE 3 Parts by Component weight Insulating Inorganic Component havingComposition 50 Powder of Table 2 Photo- Copolymer of methylmethacrylateand 50 sensitive methacrylic acid: 7 parts by weight Organic Monomer(modified trimethylolpropane Component triacrylate): 14 parts by weightPhotopolymerization initiator (2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one): 2 parts by weightSolvent (ethylcarbitol acetate): 27 parts by weight

Manufacture of Multilayered Interconnected Circuit Board

Next, a method for manufacturing a multilayered interconnected circuitboard by using the above-described photosensitive glass paste will beexplained.

First, a conductive lead is formed on an alumina substrate. Theconductive lead (a coil pattern in this embodiment) is formed by screenprinting by applying a conductive paste made by mixing and kneading 78parts by weight of an Ag powder which is a conductive component, 2 partsby weight of a glass powder and 2 parts by weight of an organic vehicle(ethylene glycol+ethyl cellulose) on the alumina substrate, drying theconductive paste, and firing the conductive paste at a temperature of800° C. in air.

Then, a photosensitive glass paste is applied on the entire surface ofthe alumina substrate provided with a conductive lead by a screenprinting technique and is then dried.

Exposure to ultraviolet light is then performed through a photomaskhaving a via hole pattern so as to expose part of the conductive lead.By curing the part on which light is irradiated, an insulating layer(glass layer) is formed.

Development is then performed in an aqueous 0.6 percent by weight Na₂CO₃solution so as to form via holes on part of the first insulating layer.Subsequently, firing at a temperature of 800° C. is performed in air soas to form a first insulating layer.

On the first insulating layer, a conductive paste identical to the oneused in formation of the conductive lead is applied by screen-printingusing a screen sheet having a desired pattern. By this process, viaholes are filled with the conductive paste, a conductive lead pattern isformed on the first insulating layer, and the circuit on the first andthe second layers are connected in series. Drying and firing steps underthe above-described conditions follow this step.

Then, for the second insulating layer, the photosensitive glass paste isapplied on the entire surface by screen-printing. Then the sameexposure, development and firing steps are repeated as those for thefirst insulating layer, so as to form the second insulating layer. Byrepeating the above-described steps, a multilayered interconnectedcircuit board having a desired number of layers is manufactured.

Evaluation

For the multilayered interconnected circuit boards having via holesmanufactured by using each of the above-described photosensitive glasspastes, the diameter of the via holes after development and afterfiring, the amount of Ag which is the conductor diffused, andsinterability are evaluated, and the results are shown in Table 4.

TABLE 4 Via Hole Via Hole Diameter after Diameter after Ag SampleDevelopment Firing Diffusion Sinterability A* 32 μm 35 μm I III B 34 μm48 μm I II C 31 μm 52 μm I I D* 32 μm 87 μm II I E 30 μm 56 μm I I F* 27μm 88 μm II I G 30 μm 55 μm I I H 27 μm 58 μm II I I* 32 μm 90 μm III IJ* 33 μm 127 μm  III I Ag Diffusion I: Slightly Diffused II: DiffusionDistance of approximately 10 mm III: Diffusion Distance of 30 mm or moreSinterability I: Excellent II: Satisfactory, No Insulation Failure III:Insulation Failure Found

As shown in Table 4, sample A having a low amount, i.e., 0.5 percent byweight, of the low melting point glass has inferior sinterability in thephotosensitive glass paste. Although shrinkage was barely observed,insulation failures were found.

All samples B, C, D, E, F, G and H containing about 1 to 30 percent byweight of the low melting point glass were sintered. Except for samplesD and F which used a high melting point glass in which the differencebetween the glass softening temperatures of the low melting point glassand that of the high melting point glass was less than about 300° C.,the diameters of the via holes did not increase significantly in any ofthese samples.

In each of the samples B, C, D, E, F, G and H containing about 1 to 30percent by weight of the low melting point glass, diffusion of Agconductor into the glass was minimized. This was because diffusion of Aginto the high melting point glass contained therein was minimized.

In sample I which is outside the scope of the present invention (contentof the low melting point glass: 35 percent by weight), not only thediameter of via holes was increased, but also diffusion of Ag into theinsulating layer was increased resulting in a decrease of resistance inthe insulating layer and insulation failure between layers. This wasmainly because the amount of the low melting point glass (in whichdiffusion of Ag is likely to occur) added to the first high meltingpoint glass, in which diffusion of Ag barely occurs, was too large.

In sample J containing a low melting point glass and no high meltingpoint glass, the diameter of the via hole was increased, the shape ofthe via hole was distorted and the conductive lead which was supposed tohave a coating thereon was instead exposed, thereby causing shortcircuiting failures between the leads when the conductor lead wasformed.

Samples D and F use a high melting point glass having a glass softeningtemperature which is not more different than about 300° C. from theglass softening temperature (T_(S)) of the low melting point glass. Insamples D and F, sintering was completed, but the via holes wereenlarged and the pattern shape was distorted. As in sample J, theconductive lead which was supposed to have a coating thereon was insteadexposed, and short circuit failure between leads occurred when theconductive leads of the second layer was formed.

Both samples C and D contain 2.5 percent by weight of the low meltingpoint glass. However, sample D does not contain the second high meltingpoint glass (B₂O₃:Al₂O₃:SiO₂=96:2.5:1.4). As a result, sample D hadincreased via hole diameter after firing compared to that of sample C.This is because sample C contained a glass having a high glass softeningpoint and the second high melting point glass remains unmelted in theglass inhibiting the viscous flow of the glass, serving as anchorage,thereby preventing the via hole diameters from increasing.

Map diagrams were obtained using WDX (wavelength-dispersed X-rayspectrometer) showing the state of Ag diffusion in sample C, which iswithin the scope of the present invention and in sample I which isoutside the scope of the present invention.

In the map diagrams, a part with a lighter color indicates a part with ahigh Ag concentration. In sample I, the sample which is outside thescope of the invention, the part with a lighter color is significantlylarge. Thus, the amount of diffused Ag is large and the insulationproperties of the insulating layer is degraded.

It is to be understood that the scope of the present invention is notlimited to the above-described embodiments and examples, and variousapplications and modifications are possible without departing from thespirit of the present invention.

What is claimed is:
 1. A photosensitive glass paste comprising: aninorganic component comprising a glass powder; and a photosensitiveorganic component, wherein the glass powder comprises: (a) about 1 to 30percent by weight of a low melting point glass powder having a glasssoftening point in the range of about 400° C. to 600° C.; and (b) about70 to 99 percent by weight of high melting point glass powder having aglass softening point at least about 300° C. higher than the glasssoftening point of the low melting point glass.
 2. A photosensitiveglass paste according to claim 1, wherein the high melting point glasspowder comprises a SiO₂—B₂O₃—K₂O glass powder in which the compositionof SiO₂, B₂O₃ and K₂O by weight is within a region surrounded by point A(65, 35, 0), point B (65, 25, 10), point C (85, 5, 10) and point D (85,15, 0) in a ternary diagram thereof.
 3. A photosensitive glass powderaccording to claim 2, wherein the high melting point glass powdercomprises a mixed powder of said SiO₂—B₂O₃—K₂O glass and aSiO₂—B₂O₃—Al₂O₃ glass having a composition of: about 93.5 to 97.8percent by weight SiO₂, about 2.0 to 5.0 percent by weight B₂O₃, andabout 0.2 to 1.5 percent by weight Al₂O₃.
 4. A photosensitive glasspaste according to claim 3, wherein the mixed powder contains about 15to 35 percent by weight of SiO₂—B₂O₃—Al₂O₃ glass relative to 100 percentby weight of the SiO₂—B₂O₃—K₂O glass.
 5. A photosensitive glass pasteaccording to claim 1, wherein the high melting point glass powdercomprises a SiO₂—B₂O₃—K₂O glass powder in which the composition of SiO₂,B₂O₃ and K₂O by weight is within a region surrounded by point E (75, 25,0), point F (75, 20, 5), point G (85, 10, 5) and point D (85, 15, 0) ina ternary diagram thereof.
 6. A photosensitive glass paste according toclaim 1, wherein the content of the inorganic component is in a range ofabout 40 to 70 percent by weight.
 7. A photosensitive glass pasteaccording to claim 6, wherein the content of the inorganic component isin a range of about 50 to 55 percent by weight.
 8. A photosensitiveglass paste according to claim 1, wherein the low melting point glasspowder has a glass melting point in a range of about 450° C. to 550° C.9. A photosensitive glass paste according to claim 1, wherein theaverage grain diameter of the high melting point glass powder and of thelow melting point glass powder are both in a range of about 0.1 to 10μm.
 10. A photosensitive glass paste according to claim 9, wherein theaverage grain diameter of the high melting point glass powder and of thelow melting point glass powder are both in a range of about 1 to 4 μm.11. A photosensitive glass paste according to claim 1, wherein about 10to 25 percent by weight of the inorganic component comprises a ceramicpowder.
 12. A photosensitive glass paste according to claim 11, whereinthe content of the inorganic component is in a range of about 40 to 70percent by weight; the low melting point glass powder has a glassmelting point in a range of about 450° C. to 550° C.; the high meltingpoint glass powder comprises a mixed powder of 100 weight parts of aSiO₂—B₂O₃—K₂O glass in which the composition of SiO₂, B₂O₃ a and K₂O byweight is within a region surrounded by point A (65, 35, 0), point B(65, 25, 10), point C (85, 5, 10) and point D (85, 15, 0) in a ternarydiagram thereof and about 15 to 35 weight parts of a SiO₂—B₂O₃—Al₂O₃glass having a composition of: about 93.5 to 97.8 percent by weightSiO₂, about 2.0 to 5.0 percent by weight B₂O₃, and about 0.2 to 1.5percent by weight Al₂O₃; and wherein the average grain diameter of thehigh melting point glass powder and of the low melting point glasspowder are both in a range of about 0.1 to 10 μm.
 13. A photosensitiveglass paste according to claim 1, wherein the content of the inorganiccomponent is in a range of about 50 to 55 percent by weight; the highmelting point glass powder comprises a mixed powder of 100 weight partsof a SiO₂—B₂O₃—K₂O glass in which the composition of SiO₂, B₂O₃ and K₂Oby weight is within a region surrounded by E (75, 25, 0), point F (75,20, 5), point G (85, 10, 5) and point D (85, 15, 0) in a ternary diagramthereof and about 20 to 30 weight parts of a SiO₂—B₂O₃—Al₂O₃ glass; andwherein the average grain diameter of the high melting point glasspowder and of the low melting point glass powder are both in a range ofabout 1 to 4 μm.